1. Field of the Invention
The invention relates to once-through steam generators including a vertical gas flue formed of tubes being disposed essentially vertically, being welded gas-tightly to one another, together forming combustion chamber walls, carrying fossil fuel burners, having an internal tube diameter, having fins on their inner surfaces forming a multiple thread with a lead and a fin height and being connected in parallel for conducting a coolant flow.
Such once-through steam generators with vertical tubing of the combustion chamber walls are less expensive to manufacture than those having helical tubing and, in addition, have a lower pressure drop on the water/steam side. However, unavoidable differences in heat supply to the individual tubes, for example due to different degrees of slagging before and after sootblowing, can lead to temperature differences of up to 160.degree. C. between individual tubes at an evaporator outlet (see Published European Application No. 0 217 079), causing damage due to inadmissible thermal stresses. Moreover, for reasons of tube cooling, it has heretofore been possible to construct such steam generators only for large unit outputs. In a publication entitled "Zwangdurchlaufkessel fur Gleitdruckbetrieb mit vertikaler Brennkammerberohrung [Once-through Forced-flow Boilers for Variable-pressure Operation with Vertical Combustion Chamber Tubing]" by H. Juzie et al. in VGB KRAFTWERKSTECHNIK 64, No. 4, starting at page 292, a lower output limit of 500 MW is given for steam generators having a combustion chamber with vertical tubing and tangential bituminous coal firing.
That publication also shows that, in addition to the internal tube diameter, the mass flow density of the coolant in the tube is a parameter which determines the rheological structure of the parallel tube system acting as the evaporator heating surface. Typical mass flow densities are between 2000 and 3000 kg/m.sup.2 s for helical tubing of the combustion chamber with tubes that are smooth on the inside, and between 1500 and 2000 kg/m.sup.2 s for vertical tubing with internally finned tubes. With those construction parameters, the portion or share of the frictional pressure drop in the total pressure drop of the once-through evaporators is very high. Consequently, such evaporators have a typical characteristic, starting from the construction state, according to which the mass throughput in an individual tube decreases with more intensive heating thereof and increases with less intensive heating thereof.
That characteristic is one cause of major temperature differences between individual tubes at the evaporator outlet in gas flues having vertically disposed tubes. In order to reduce such temperature differences, it is known to incorporate restrictors at the evaporator inlet and/or to provide mixing headers, into which the tubes lead and in which a certain enthalpy equalization takes place by means of mixing, in the upper part of the combustion chamber walls outside the gas flue. At unit outputs below 500 MW, helical tubing for the combustion chamber walls has been provided in heretofore constructed once-through steam generators, in order to be able to maintain the mass flow density in the tubes necessary for cooling the smooth tubes and in order to achieve a certain equalization of heating along the large tube length. However, that measure causes higher manufacturing costs of the once-through steam generators and requires relatively high feed pump powers because of the high pressure drop which occurs.